Conventional awnings are usually made of canvas having a thick texture, which may be unwound from a roll and extended by mechanical means such as lateral swivel arms. Winding back such awnings presents no special problems, as the required force depends essentially upon the dimensions of the awning and of the devices provided for holding the lower free edge of the awning at some distance from the vertical plane of the wind-up roll axis.
These conventional awnings have a rather low heat-insulating power, which is supplied only by the thickness of the fabric. Even when using thick canvas, the temperature difference between air outside of the shaded area provided by the awning and air under the awning remains quite small, and the shadow cast by the awning does not lower the temperature as much as might be expected. This effect is usually a combined result of the high temperature of ambient air and of the very substantial heat transfer across the textile fabric. In fact, in the absence of air circulation, particularly with awnings having lateral trimmings such as hooded awnings some very hot air will accumulate directly under the awning, so that instead of feeling cool under the shade of the awning an unpleasant hot feeling is experienced similar to being in a hothouse. This effect is even observed at times of intense sunshine when there is some air circulation and when the awning has no side trimmings. The thickness of the hot air layer will then be reduced, but since the thermal radiation under the awning remains quite strong, the effectiveness of the awning is not satisfactory.